Thermochromism, Franck-Condon analysis and interfacial dynamics of a donor-acceptor copolymer with a low band gap

The electronic properties of the donor-acceptor (DA) polymer poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT) have been investigated using spectroscopic and computational techniques. Electronic absorption and emission spectra reveal the presence of an ordered and a disordered phase in solution. Franck-Condon modeling of the ordered phase yields Huang-Rhys factors of 0.55 (20 °C) and 0.51 (-180 °C), indicating little structural distortion between ground and excited state. DFT calculations with resonance Raman spectroscopy are consistent with a lowest energy excited state that is electronically delocalized and has little charge-transfer character, unexpected for a copolymer with a low bandgap (∼1.8 eV). Transient absorption spectroscopy of PTBT:fullerene blends reveals near-unity internal charge-transfer yields in both ordered and disordered film morphologies. In the disordered blend, charge transfer is complete within the laser pulse (100 fs), whereas the ordered blend also features a slower phase due to exciton diffusion in the phase separated morphology. In the ordered blend, the spectra and dynamics of charge transfer reveal that excitons and charges promptly occupy delocalized states on extended polymer chains. The pervasive use of donor-acceptor structures in polymer devices makes understanding the interplay of morphology and electronic structure of these polymers essential and here a spectroscopic and computational investigation gives an extensive picture of the electronic properties and their effect on charge dynamics in a DA polymer.